Lithographic Apparatus and Device Manufacturing Method Using Dose Control

ABSTRACT

A system and method are used to manufacture a device using at least one exposure step. Each exposure step projects a patterned beam of radiation onto a substrate. The patterned beam includes a plurality of pixels. Each pixel delivers a radiation dose no greater than a predetermined normal maximum dose to the target portion in the exposure step and/or at least one selected pixel delivers an increased radiation dose, greater than the normal maximum dose. The increased dose may be delivered to compensate for the effect of a defective element at a known position in the array on a pixel adjacent a selected pixel or compensate for underexposure of the target portion at the location of a selected pixel resulting from exposure of that location to a pixel affected by a known defective element in another exposure step.

This application incorporates by reference in their entireties U.S.patent application Ser. No. 11/580,134, filed Oct. 13, 2006 and U.S.patent application Ser. No. 10/862,876, filed Jun. 8, 2004.

BACKGROUND

1. Field of the Invention

The present invention relates to a lithographic apparatus and a devicemanufacturing method.

2. Related Art

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. The lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs), flatpanel displays, and other devices involving fine structures. In aconventional lithographic apparatus, a patterning means, which isalternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern corresponding to an individual layer of theIC (or other device), and this pattern can be imaged onto a targetportion (e.g., comprising part of one or several dies) on a substrate(e.g., a silicon wafer or glass plate) that has a layer ofradiation-sensitive material (e.g., resist). Instead of a mask, thepatterning means may comprise an array of individually controllableelements that generate the circuit pattern.

In general, a single substrate will contain a network of adjacent targetportions that are successively exposed. Known lithographic apparatusinclude steppers, in which each target portion is irradiated by exposingan entire pattern onto the target portion in one go, and scanners, inwhich each target portion is irradiated by scanning the pattern throughthe projection beam in a given direction (the “scanning” direction),while synchronously scanning the substrate parallel or anti-parallel tothis direction.

Maskless lithography refers to lithography that uses an array ofindividually controllable elements instead of a mask to form a desiredradiation exposure pattern on a target. The array of elements is used topattern a radiation beam, and the patterned beam is projected onto atarget surface of a substrate. The projected pattern comprises aplurality of pixels, each pixel typically corresponding to a respectiveone of the array of controllable elements. Generally, in suchtechniques, each pixel has a peak intensity that is primarily dependentupon the corresponding respective element, but which is also dependentto some degree on the elements adjacent to the corresponding element.

In a basic form of programmable array, each element may be controllableto adopt one of two states: a “black” state, in which its correspondingpixel on the projective pattern has a minimum intensity; and a “white”state, in which the corresponding pixel has maximum intensity. Thus, thearray can be controlled to expose the target portion of the substrate toa desired pattern of corresponding “black” and “white” pixels, eachpixel delivering a corresponding dose of radiation during the exposurestep.

It is also known to use more sophisticated programmable arrays, in whicheach element is controllable to adopt a plurality of grey states, inaddition to the black and white states. This allows pixels to deliverdoses between the “white” maximum and “black” minimum. This finer dosecontrol delivered by each pixel enables finer features to be achieved inthe eventual exposure pattern.

In the art, the process of exposing the substrate target surface to aplurality of pixels is sometimes referred to as a printing step, andwhen an array of elements with black, white, and grey states is used,the process may be described as grey-scale printing.

In certain applications, the radiation dose pattern to which thesubstrate is to be exposed may be described as comprising “white”regions, defined as regions to which a dose greater than a certain valueis to be delivered, and “black” regions, defined as regions to whichdoses smaller than a certain value are to be delivered. For example, thesubstrate may have a resist layer, the resist material having a certainthreshold activation dose. In such cases, the white regions are those towhich the delivered dose is to exceed the activation threshold, and theblack regions are to receive less than the activation threshold dose,such that on subsequent development, the black regions are removed,leaving only the pattern of white regions.

When controllable element arrays are used in maskless lithography, thereis a possibility that one or more of the elements may be, or may become,defective, and will be unresponsive to control signals, or will notrespond in the normal, desired way. For example, a defective element maybe an unresponsive element, stuck in a black or grey state.Alternatively, it maybe an element controllable to adopt only a reducednumber of its normal states, so that its white state or whitest states,are inaccessible.

When no compensation is made for the defective elements, radiation dosesdelivered to the target substrate may be smaller than desired. “White”dead pixels may also arise, for example corresponding to elements stuckin a fully white state. “White” dead pixels cannot be corrected whenintended to print “black.” Therefore, all “white” dead pixels need to bemade “black” before the array is used for lithography, for example, inthe case of programmable mirror arrays, by mechanically deforming themto a tilted position by micromanipulation, by removing them, by creatinga grating on them, or by coating the mirror black by local deposition ofan absorbing material.

To produce a desired exposure pattern on a target substrate, it is knownto use a two-pass maskless lithography method. In such a method, eachpart of the target surface is exposed to a pixel twice, i.e., twoexposure steps are used to deliver, in combination, the total requiredradiation dose to each part of the substrate. Typically, the substrateis moved between the first and second exposure steps, relative to thebeam projection system, such that a particular part of the targetsurface is not exposed to the same pixel twice (i.e., a pixelcorresponding to the same controllable element). This has been done tolimit the maximum effect that a defective pixel can have. Also, even ifno defective elements are present on the array, a two-pass systemenables an improved total dose accuracy to be achieved compared with asingle exposure method.

To produce a desired exposure pattern of “white” and “black” regions ona target surface of a resist layer of a substrate, a two-pass system istypically arranged such that in each exposure step a fully white pixeldelivers a radiation dose just greater than half the resist activationthreshold dose. This is achieved by appropriate selection of exposuretime (i.e., the time for which the substrate is exposed to a particularpixel in each exposure step) and the intensity of the radiation source.

Previous systems tried to use as short exposure times as possible, whichincreases pixel printing rate, and hence improves throughput, but thisis limited by the switching speeds of the controllable elements (i.e.,how fast they can be controlled to switch from one state to another).Previous systems also tried to use a radiation source whose output poweris no higher than necessary. This is because, generally, the higher thesource power the higher its cost, and the higher the cost of the beamconditioning, transport, and projection systems required to accommodatethe beam. In addition, higher beam intensity can lead to an increasedrate of degradation of certain components. Thus, in previous systems,the general requirement has been that the projected beam should interactwith a fully “white” element to produce a corresponding “white” pixel,which delivers the required dose in the in particular exposure period,i.e., a dose just greater than half the resist threshold dose.

To make best use of available source power, prior art methods have beenarranged to print the “white” regions of the target with elements set totheir fully “white” states.

Thus, in general, in a typical two-pass method, “white” regions of thetarget have been printed using “white” pixels of maximum, 100% intensity(i.e., the maximum achievable intensity with the particular radiationsource and range of element states), and “grey” and “black” pixels, withintensities down to 0% have also been used to build up the desireddosage pattern.

In such methods, problems occur if an element is completely unresponsiveand set in a “black” state, or is otherwise unable to interact with thesource beam to make a contribution to its corresponding pixel. In otherwords, if the element is a “black dead element” and its correspondingpixel is a “black dead pixel.” If, for example, the “black” dead pixelfalls on a “white” region of the target surface, then instead ofdelivering the required dose (e.g., approximately half the thresholddose) in a first pass it will deliver a much reduced dose, even a zerodose. Even though the part of the “white” region upon which the “black”dead pixel falls may be exposed to a full, 100% intensity “white” pixelin the second pass (e.g., corresponding to a non-defective element inthe same element array, or a non-defective element in another array),the combined dose it receives may thus fall significantly short of thethreshold dose required.

Clearly, such under exposure resulting from defective elements has adetrimental effect on the dosage pattern achieved by the process. Itwill be appreciated that similar problems also occur if, rather thanbeing completely “black,” a defective element is set in a “grey” stateand is unable to deliver, via its corresponding pixel, a sufficientlyhigh dosage in one of the passes.

One previous attempt to compensate for a dead black pixel is illustratedin FIG. 2. Here, a simplified projected pattern 1 of nine pixels isshown, falling on a corner of a white region of a target surface. Theboundary of the white region is indicated by broken line 13. Pixels 10,11 falling on the white region are intended to be fully white (i.e., toprint at full intensity), and pixels 12 falling on the black region areintended to be fully black (i.e., zero intensity). However, pixel 11 isa dead black pixel.

To compensate for this, rather than being fully black, non-white pixels12 neighboring dead black pixel 11 are made grey, such that theircontributions to the radiation dosage delivered by pixel 11 combine tocompensate at least partially for the dosage lost as a result of thedefective element. Thus, the neighboring black pixels in the sameexposure step (e.g., write pass) have been used to compensate for a deadblack pixel 11 falling on the edge of a white region, and it is alsoknown to use neighboring (i.e., surrounding) black pixels to providesuch compensation in a preceding or subsequent pass.

A problem with this compensation method, however, is that by increasingthe intensities of pixels 12 from black to grey values, the positions ofthe feature edges between the black and non-defective white pixels maybe undesirably shifted, and once a black region has been given a greydosage, it is not possible to reverse this. Correcting with neighboringpixels in this way leads not only (and necessarily) to a shift of theedge position, it also makes the edge less steep. The (N)ILS (which isthe (normalized) imaging log slope) gets worse. Also, if the dead blackpixel falls on a line edge, rather than at a corner, the number ofimmediately adjacent black pixels for compensation purposes is reduced.Furthermore, if a dead black pixel falls within a white region such thatit is surrounded by non-defective white pixels, the above technique canprovide no compensation.

Another attempt to solve the problem of underexposure resulting fromdefective elements/pixels has been to use an additional write pass,which may be referred to as a “clean-up pulse” or exposure. Here, thesubstrate is moved with respect to the projection system such that nopart of the substrate can be exposed to the same defective pixel twice.The clean-up pass is made specifically to deliver targeted radiationdoses to selected parts of the substrate which received lower than theirdesired doses in the preceding exposure step or steps. Although goodcompensation may be achieved, the problem with this technique is thatthe need for an additional write pass reduces throughput, or increasescosts and complexity if it is achieved by adding further arrays ofcontrollable elements to those normally required for printing.

Although two-pass systems have been described, it will be appreciatedthat problems of defective elements occur also in single pass methods,and multiple-pass methods using three or more passes to achieve requiredradiation dosage patterns.

Thus, there remain problems associated with the compensation for effectsof defective elements in maskless lithography.

Therefore, what is needed is lithographic methods and apparatus thatallow for more efficient and effective compensation for effects ofdefective elements in maskless lithography.

SUMMARY OF THE INVENTION

According to an embodiment of the invention, there is provided a devicemanufacturing method comprising the steps of providing a projection beamof radiation using an illumination system, using an array ofindividually controllable elements to impart the projection beam with apattern in its cross-section, projecting the patterned beam of radiationonto a target portion of a substrate, wherein the projected radiationpattern comprises a plurality of pixels, such that no compensation fordefective elements is required. The elements are controlled such thateach pixel delivers a radiation dose no greater than a predeterminednormal maximum dose to the target portion in the exposure step. Whencompensation for defective elements is required, the method controls theelements such that at least one selected pixel delivers an increasedradiation dose, greater than the normal maximum dose, in the exposurestep. This compensates at least partially for at least one of: (a) theeffect, in the same exposure step, of a defective element, at a knownposition in the array, on a pixel adjacent a selected pixel and (b) anunderexposure of the target portion at the location of a selected pixelresulting from exposure of the location to a pixel affected by a knowndefective element in another (i.e., a different) exposure step. The stepof exceptionally controlling the elements may also be described as acompensation step.

As discussed above, conventional compensation methods were limited tousing compensation doses up to the normal printing maximum, i.e., thedose provided by a fully-white, non-defective pixel in a single exposurestep/write pass. Thus, compensation was limited to increasing the dosedelivered by selected pixels in the projected pattern that wouldotherwise have been black or grey.

In contrast, according to this embodiment of the present invention,doses up to the predetermined normal maximum for normal printing areused, but the method reserves at least one increased dose forcompensation purposes. This can allow for that nominal white pixels inthe projected radiation pattern to be used for compensation purposes.Thus, even when a dead black pixel falls in the middle of a group ofsurrounding white pixels it can be compensated, in the same exposurestep, by increasing the radiation dose delivered by one or more of thoseneighboring white pixels, above the normal fully white value. This mayconveniently be achieved by reserving one or more of the elements' mostintense states (i.e., the states in which they interact with theprojected beam to make the greatest contributions to their pixel'sintensities) only for compensation purposes, the lower intensity statesbeing used for normal printing.

In a multi-pass (i.e., a multiple exposure step) example, an increaseddose may be delivered in one step to a location that has beenunderexposed in a previous step, or that will receive a reduced dose ina subsequent pass as a result of being exposed to the pixelcorresponding to a known defective element. The increased dose may, incertain embodiments, be large enough to completely compensate, in asingle step, for any degree of underexposure in another step.Compensation for the effects, in a particular step, of a defective pixelin the same step, is not then required.

In general, in a multi-step method, as much compensation as possible ismade in each step for the effects of defective elements in that stepusing adjacent pixels. This reduces the amount of compensation to beprovided by the other step or steps.

Compensation in the same exposure step, using adjacent pixels, mayconveniently be referred to as simultaneous compensation, whilstcompensation in preceding and subsequent steps may be referred to aspre-compensation and post-compensation respectively.

Compared with conventional methods, if the above embodiment produces thesame final radiation dosage pattern on a substrate in the same time withthe same number of passes/exposure steps, then it may employ anillumination system that delivers a more intense beam. The increasedintensity is required to provide the capacity to deliver the radiationdoses above the normal printing maximum. Although there areconsequential increases in cost, significant improvements to the patternquality are achieved as a result of the ability to compensate with whitepixels, and a further advantage is the avoidance of an additionalclean-up pass, which would reduce throughput.

In one example, an increased radiation dose deliverable in acompensation step is larger than the normal maximum dose by a factor ofup to at least about 1.1, 1.5, or even 2. In the last case, in atwo-pass system, a compensation dose in one exposure step can providecomplete compensation for a zero dose, rather than a normal white dose,being delivered by a dead black pixel in the other pass. If the maximumincreased dose is less than twice the normal maximum dose then somesimultaneous compensation is required, in addition to pre- orpost-compensation for full compensation.

Typically, a substrate will have a target surface to which apredetermined radiation dosage pattern is to be delivered. In thisexample, the dosage pattern comprises nominal white regions, to which aradiation dosage at least equal to a predetermined threshold value is tobe delivered, and nominal black regions, to which a radiation dosageless than the predetermined threshold value is to be delivered. The stepof ordinarily controlling the elements (i.e., the normal printing step)will then comprise controlling the elements such that each pixelprojected onto (i.e., falling on) a white region delivers a radiationdose no greater than the predetermined normal maximum dose in theexposure step. The compensation step then comprises controlling theelements such that each of the selected pixels is projected onto a whiteregion and delivers an increased radiation dose to that white region. Inother words, compensation involves the white regions selectively beinggiven overdoses, above the normal white printing threshold. A pixelfalling on a white region may thus be selected to deliver an increaseddose, of calculated magnitude, to compensate for an adjacent dead pixelfalling on the same white region in the same exposure step.Alternatively, or additionally, a pixel may be selected to deliver anincreased dose to a white region at a location underexposed by adefective pixel in a preceding or subsequent exposure step.

In one example, a substrate may comprise a layer of radiation sensitivematerial (e.g., resist) having an activation threshold, and thepredetermined threshold value may be equal to that activation threshold.In such instances, the target surface is a surface of the layer. For asingle-pass example, the normal maximum dose is arranged so as to begreater than the predetermined threshold value. For a multi-passexample, the normal maximum dose may be less than the predeterminedthreshold value and greater than half the predetermined threshold value.

In one example, the method may comprise two of the exposure steps, thetwo exposure steps combining to deliver the predetermined radiationdosage pattern to a common target portion of the target surface. Thesame array of elements may be used in each step, but with different setsof pixels, corresponding to different sets of the controllable elements,being used to expose the common target portion. Alternatively, differentarrays may be used. As described above, each exposure step may comprisesubstantially simultaneous compensation for the effects of defectiveelements on that step. Additionally, or alternatively, the firstexposure step may comprise a pre-compensation step for underexposure inthe second exposure step, and the second exposure step may comprisepost-compensation for underexposure effects in the first exposure step.

In one example, each pixel in the projected radiation patterncorresponds to a respective one of the elements of the array. Thus, thestep of exceptionally controlling the elements may comprise controllingselected elements such that their corresponding pixels deliver increasedradiation doses.

In one example, the array of elements includes a defective element at aknown position. In this example, the compensation step may comprisecontrolling one or more elements immediately adjacent the defectiveelement such that their corresponding pixels, immediately adjacent thedefective pixel (i.e. the pixel corresponding to the defective element),deliver increased radiation doses. Thus, when a defective pixel falls ona white region, its effect may be compensated by controlling elementssurrounding the defective element such that surrounding pixels, whichfall on the same white region, deliver increased doses.

It will be appreciated that, for substantially simultaneous compensationto be provided, each pixel in the projected radiation pattern will havean intensity distribution primarily dependent on a respectivecorresponding element but also dependent on elements immediatelyadjacent the respective corresponding element in the array.

In various examples, dose control may be achieved in a variety of ways.For example, pixel intensity may be adjusted by controlling elementstates and/or by controlling projection beam intensity (e.g., byadjusting laser pulse height, given a certain pulse length, such as10-30 ns, when the projection beam is provided by a pulsed lasersource). Alternatively, pixel duration may be adjusted, for example byadjusting laser pulse length.

In one example, control of the radiation doses delivered by each pixelis achieved by controlling (adjusting) pixel intensity. Thus, the stepof ordinarily controlling the elements may comprise controlling theelements such that each pixel has a peak intensity no higher than apredetermined normal maximum intensity, and the step of exceptionallycontrolling the elements may comprise controlling the elements such thateach selected pixel has an increased peak intensity, higher than thenormal maximum intensity. The increased peak intensity may be higherthan the normal maximum intensity by up to at least a factor of about1.1, 1.5, or even 2, such that an intensity “headroom” of 100-200% isreserved for compensation purposes, above the 0-100% used for normalprinting.

In one example, arrays of elements are used in which each element, whennot defective, is controllable to selectively adopt one of at leastthree states. These states comprising: (a) a nominal black state, inwhich the element interacts with the projection beam so as to make aminimum contribution to the radiation dose delivered by a correspondingpixel; (b) at least one nominal grey state, in which the elementinteracts with the projection beam so as to make an increasedcontribution to the radiation dose delivered by the corresponding pixel;and (c) at least one nominal white state, in which the element interactswith the projection beam so as to make a contribution to the radiationdose delivered to the corresponding pixel greater than that in any greystate. Normal printing is then performed using elements set in the blackor grey states, and compensation is performed using elements in whitestates.

In one example, controllable arrays, such as programmable mirror arrays,are used in which each mirror element is controllable to adopt a rangeof tilt angles, such that the state of an element can be used todetermine the dosage delivered by a corresponding pixel by affecting theintensity of that pixel.

Although a method embodying the invention may be carried out usingelements which have a single black state, a single grey state, and asingle white state, each non-defective element can also be controllableto selectively adopt one of a series of grey states and a series ofwhite states, each grey or white state corresponding to a respectivecontribution to a corresponding pixel's peak intensity. The grey andwhite states may form a discrete or a continuous series. Preferably,each non-defective element may be controllable to adopt as many as 64states, or more, to enable greyscale printing to be achieved. Hence,independence between the position of the projected radiation pattern andthe “grid” defined by the array of controllable elements may beachieved.

In one example, the series of white states includes a whitest state inwhich the element interacts with the beam to provide a maximumcontribution to the peak intensity of the corresponding pixel, themaximum contribution being at least twice as large as the largestcontribution corresponding to a grey state. In other words, an elementin the whitest state may provide a pixel having twice the intensity asthat of a pixel corresponding to an element in the most intense greystate.

In this example, the black and grey states together form a first set ofstates, used for normal beam patterning and exposure of the targetsurface of the substrate, and the white states form a second set ofstates, reserved for use in compensating for the effects of defectiveelements in the current exposure step or in previous or subsequentexposure steps. Reserving the second set of states in this way providesexposure headroom.

Another embodiment of the present invention provides a lithographicapparatus comprising an illumination system for supplying a projectionbeam of radiation, an array of individually controllable elementsserving to impart the projection beam with a pattern in itscross-section, a controller arranged to control the elements, asubstrate table for supporting a substrate, and a projection system forprojecting the patterned beam onto a target portion of the substrate.The projected radiation pattern comprises a plurality of pixels, eachpixel delivering a respective radiation dose to the target portion. Eachelement, when not defective, is controllable to selectively adopt one ofat least three states. The states comprise: (a) a nominal black state,in which the element interacts with the projection beam so as to make aminimum contribution to the radiation dose delivered by a correspondingpixel; (b) at least one nominal grey state, in which the elementinteracts with the projection beam so as to make an increasedcontribution to the radiation dose delivered by the corresponding pixel;and (c) at least one nominal white state, in which the element interactswith the projection beam so as to make a contribution to the radiationdose delivered to the corresponding pixel greater than that in any greystate. The controller is arranged ordinarily to control the elementssuch that each element adopts one of the black or grey states. Thecontroller is further arranged to selectively control the elements toadopt white states to provide compensation for the effects of defectivepixels.

In this embodiment, the controller is arranged to use the black and greystates for ordinary, normal substrate exposure, and to reserve the whitestates for compensation purposes, i.e. only uses them when a defectiveelement has affected, is affecting, or will be affecting the dosepattern delivered. Each non-defective element is controllable toselectively adopt one of a series of grey states, and one of a series ofwhite states, and the elements may conveniently be provided by aprogrammable mirror array.

In one example, there is a lithographic apparatus in combination with asubstrate having a target surface to which a predetermined radiationdosage pattern is to be delivered. The dosage pattern comprises nominalwhite regions, to which a radiation dosage at least equal to apredetermined threshold value is to be delivered, and nominal blackregions, to which a radiation dosage less than the predeterminedthreshold value is to be delivered. The series of grey states may theninclude a maximum grey state in which the increased contribution is amaximum for the grey states. The illumination system and controllerbeing arranged to expose the target portion to each pixel for a commonexposure time, and the illumination system and elements being arrangedsuch that exposure of the target portion for the common exposure time toa pixel whose corresponding element is in the maximum gray statedelivers a dose at least equal to the predetermined threshold value.Alternatively, exposure of the target portion for the common exposuretime to a pixel whose corresponding element is in the maximum gray statemay be arranged to deliver a dose less than the predetermined thresholdvalue but greater than half the predetermined threshold value.

The above embodiments can allow for better CD (charge dosage) controlthan with uncorrected dead pixels or than with conventional correctionmethods using surrounding pixels limited to 100% normal whiteness only.

The above embodiment can be described as providing the capability toprint “whiter-than-white” for correction purposes.

Another embodiment can prevent a third pass being needed to print amissed dose in a clean-up scheme, and hence avoid the consequentialreduction in throughput when using the same number of controllableelement arrays (33%) or the additional cost for the element arrays,associated electronics, and wider projector field that would result ifan additional element row were needed to provide clean-up doses.

Further embodiments, features, and advantages of the present inventions,as well as the structure and operation of the various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1 depicts a lithographic apparatus, according to one embodiment ofthe invention.

FIG. 2 illustrates a defective pixel compensation method.

FIG. 3 illustrates part of a lithographic method, according to oneembodiment of the present invention.

FIG. 4 illustrates defective pixel compensation steps used in a methodaccording to one embodiment of the present invention.

FIG. 5 illustrates further defective pixel compensation steps used in amethod according to one embodiment of the present invention.

FIG. 6 illustrates part of an array of elements and the correspondingintensity pattern produced on a target substrate in a lithographicmethod.

FIG. 7 illustrates part of an array of elements and the correspondingintensity pattern produced on a target substrate in a lithographicaccording to one embodiment of the present invention.

FIG. 8 illustrates part of an array of elements and the correspondingintensity pattern produced on a target substrate in a lithographicmethod including a compensation step according to one embodiment of thepresent invention.

The present invention will now be described with reference to theaccompanying drawings. In the drawings, like reference numbers mayindicate identical or functionally similar elements.

DETAILED DESCRIPTION OF THE EMBODIMENTS Overview and Terminology

In one embodiment of the present invention using a microlens arrayimaging system, it is the function of a field lens of a beam expander(which field lens may be formed from two or more separate lenses) in anillumination system to make a projection system telecentric by ensuringthat all components of the light beam between the field lens and themicrolens array are parallel and perpendicular to the microlens array.However, although light beams between the field lens and the microlensarray may be substantially parallel, absolute parallelism may not beachievable.

Thus, given a degree of non-telecentricity in the projection system,according to one embodiment of the present invention, smallmagnification adjustments can be achieved without undue loss of focus bydisplacing one or more of the lens components which are located betweenthe pupil and the substrate table.

In one embodiment, a projection system will define a pupil. The term“pupil” being used in this document to refer to a plane where rays ofthe projection beam intersect which rays leave the patterning systemfrom different locations relative to the patterning system but at thesame angle relative to an axis of the projection beam which is normal tothe patterning system.

For example, according to one embodiment of the present invention,assuming a microlens imaging system in which the field lens is initiallyarranged to generate a perfectly parallel beam of radiation betweenitself and the array of lenses. Also, assume that light reaching thefield lens is diverging. Any displacement of the field lens away fromthe microlens array will result in the projection beam becoming slightlydivergent, whereas displacement of the field lens towards the microlensarray will result in the projection beam becoming slightly convergent.Given however that the field lens is a relatively weak lens,displacements necessary to change the magnification of the projectionsystem to compensate for distortions of the substrate (typically of theorder of parts per million) can be achieved without affecting the focusof the projection beam on the substrate surface to an unacceptableextent. Although focus change due to displacement of the microlens arraytowards or away from the substrate is a first order effect, andresultant changes in magnification on a second order effect,nevertheless useful magnification adjustment may be made.

In this embodiment, the field lens may be made up of a single or two ormore lenses. Each field lens may be simply moved in translation eithertowards or away from the microlens array, or the field lens may betilted so as to result in a differential change in magnification acrossthe surface of the exposed substrate. Similarly, the microlens array maybe moved in translation and/or tilted.

The term “array of individually controllable elements” as here employedshould be broadly interpreted as referring to any device that can beused to endow an incoming radiation beam with a patterned cross-section,so that a desired pattern can be created in a target portion of thesubstrate. The terms “light valve” and “Spatial Light Modulator” (SLM)can also be used in this context. Examples of such patterning devicesare discussed below.

A programmable mirror array may comprise a matrix-addressable surfacehaving a viscoelastic control layer and a reflective surface. The basicprinciple behind such an apparatus is that, for example, addressed areasof the reflective surface reflect incident light as diffracted light,whereas unaddressed areas reflect incident light as undiffracted light.Using an appropriate spatial filter, the undiffracted light can befiltered out of the reflected beam, leaving only the diffracted light toreach the substrate. In this manner, the beam becomes patternedaccording to the addressing pattern of the matrix-addressable surface.

It will be appreciated that, as an alternative, the filter may filterout the diffracted light, leaving the undiffracted light to reach thesubstrate. An array of diffractive optical micro electrical mechanicalsystem (MEMS) devices can also be used in a corresponding manner. Eachdiffractive optical MEMS device can include a plurality of reflectiveribbons that can be deformed relative to one another to form a gratingthat reflects incident light as diffracted light.

A further alternative embodiment can include a programmable mirror arrayemploying a matrix arrangement of tiny mirrors, each of which can beindividually tilted about an axis by applying a suitable localizedelectric field, or by employing piezoelectric actuation means. Onceagain, the mirrors are matrix-addressable, such that addressed mirrorswill reflect an incoming radiation beam in a different direction tounaddressed mirrors; in this manner, the reflected beam is patternedaccording to the addressing pattern of the matrix-addressable mirrors.The required matrix addressing can be performed using suitableelectronic means.

In both of the situations described here above, the array ofindividually controllable elements can comprise one or more programmablemirror arrays. More information on mirror arrays as here referred to canbe gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193,and PCT patent applications WO 98/38597 and WO 98/33096, which areincorporated herein by reference in their entireties.

A programmable LCD array can also be used. An example of such aconstruction is given in U.S. Pat. No. 5,229,872, which is incorporatedherein by reference in its entirety.

It should be appreciated that where pre-biasing of features, opticalproximity correction features, phase variation techniques and multipleexposure techniques are used, for example, the pattern “displayed” onthe array of individually controllable elements may differ substantiallyfrom the pattern eventually transferred to a layer of or on thesubstrate. Similarly, the pattern eventually generated on the substratemay not correspond to the pattern formed at any one instant on the arrayof individually controllable elements. This may be the case in anarrangement in which the eventual pattern formed on each part of thesubstrate is built up over a given period of time or a given number ofexposures during which the pattern on the array of individuallycontrollable elements and/or the relative position of the substratechanges.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as, for example, the manufacture of DNA chips,MEMS, MOEMS, integrated optical systems, guidance and detection patternsfor magnetic domain memories, flat panel displays, thin film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist) or a metrology or inspection tool.Where applicable, the disclosure herein may be applied to such and othersubstrate processing tools. Further, the substrate may be processed morethan once, for example in order to create a multi-layer IC, so that theterm substrate used herein may also refer to a substrate that alreadycontains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUV) radiation (e.g. having a wavelength in the range of5-20 nm), as well as particle beams, such as ion beams or electronbeams.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection systems, includingrefractive optical systems, reflective optical systems, and catadioptricoptical systems, as appropriate, for example, for the exposure radiationbeing used, or for other factors such as the use of an immersion fluidor the use of a vacuum. Any use of the term “lens” herein may beconsidered as synonymous with the more general term “projection system.”

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the projection beam ofradiation, and such components may also be referred to below,collectively or singularly, as a “lens.”

The lithographic apparatus may be of a type having two (e.g., dualstage) or more substrate tables (and/or two or more mask tables). Insuch “multiple stage” machines the additional tables may be used inparallel, or preparatory steps may be carried out on one or more tableswhile one or more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index (e.g.,water), so as to fill a space between the final element of theprojection system and the substrate. Immersion liquids may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the first element of the projection system.Immersion techniques are well known in the art for increasing thenumerical aperture of projection systems.

Further, the apparatus may be provided with a fluid processing cell toallow interactions between a fluid and irradiated parts of the substrate(e.g., to selectively attach chemicals to the substrate or toselectively modify the surface structure of the substrate).

Lithographic Projection Apparatus

FIG. 1 schematically depicts a lithographic projection apparatus 100according to an embodiment of the invention. Apparatus 100 includes atleast a radiation system 102, an array of individually controllableelements 104, an object table 106 (e.g., a substrate table), and aprojection system (“lens”) 108.

Radiation system 102 can be used for supplying a projection beam 110 ofradiation (e.g., UV radiation), which in this particular case alsocomprises a radiation source 112.

An array of individually controllable elements 104 (e.g., a programmablemirror array) can be used for applying a pattern to projection beam 110.In general, the position of the array of individually controllableelements 104 can be fixed relative to projection system 108. However, inan alternative arrangement, an array of individually controllableelements 104 may be connected to a positioning device (not shown) foraccurately positioning it with respect to projection system 108. As heredepicted, individually controllable elements 104 are of a reflectivetype (e.g., have a reflective array of individually controllableelements).

Object table 106 can be provided with a substrate holder (notspecifically shown) for holding a substrate 114 (e.g., a resist coatedsilicon wafer or glass substrate) and object table 106 can be connectedto a positioning device 116 for accurately positioning substrate 114with respect to projection system 108.

Projection system 108 (e.g., a quartz and/or CaF2 lens system or acatadioptric system comprising lens elements made from such materials,or a mirror system) can be used for projecting the patterned beamreceived from a beam splitter 118 onto a target portion 120 (e.g., oneor more dies) of substrate 114. Projection system 108 may project animage of the array of individually controllable elements 104 ontosubstrate 114. Alternatively, projection system 108 may project imagesof secondary sources for which the elements of the array of individuallycontrollable elements 104 act as shutters. Projection system 108 mayalso comprise a micro lens array (MLA) to form the secondary sources andto project micro spots onto substrate 114.

Source 112 (e.g., an excimer laser) can produce a beam of radiation 122.Beam 122 is fed into an illumination system (illuminator) 124, eitherdirectly or after having traversed conditioning device 126, such as abeam expander 126, for example. Illuminator 124 may comprise anadjusting device 128 for setting the outer and/or inner radial extent(commonly referred to as a-outer and a-inner, respectively) of theintensity distribution in beam 122. In addition, illuminator 124 willgenerally include various other components, such as an integrator 130and a condenser 132. In this way, projection beam 110 impinging on thearray of individually controllable elements 104 has a desired uniformityand intensity distribution in its cross section.

It should be noted, with regard to FIG. 1, that source 112 may be withinthe housing of lithographic projection apparatus 100 (as is often thecase when source 112 is a mercury lamp, for example). In alternativeembodiments, source 112 may also be remote from lithographic projectionapparatus 100. In this case, radiation beam 122 would be directed intoapparatus 100 (e.g., with the aid of suitable directing mirrors). Thislatter scenario is often the case when source 112 is an excimer laser.It is to be appreciated that both of these scenarios are contemplatedwithin the scope of the present invention.

Beam 110 subsequently intercepts the array of individually controllableelements 104 after being directing using beam splitter 118. Having beenreflected by the array of individually controllable elements 104, beam110 passes through projection system 108, which focuses beam 110 onto atarget portion 120 of the substrate 114.

With the aid of positioning device 116 (and optionally interferometricmeasuring device 134 on a base plate 136 that receives interferometricbeams 138 via beam splitter 140), substrate table 106 can be movedaccurately, so as to position different target portions 120 in the pathof beam 110. Where used, the positioning device for the array ofindividually controllable elements 104 can be used to accurately correctthe position of the array of individually controllable elements 104 withrespect to the path of beam 110, e.g., during a scan. In general,movement of object table 106 is realized with the aid of a long-strokemodule (course positioning) and a short-stroke module (finepositioning), which are not explicitly depicted in FIG. 1. A similarsystem may also be used to position the array of individuallycontrollable elements 104. It will be appreciated that projection beam110 may alternatively/additionally be moveable, while object table 106and/or the array of individually controllable elements 104 may have afixed position to provide the required relative movement.

In an alternative configuration of the embodiment, substrate table 106may be fixed, with substrate 114 being moveable over substrate table106. Where this is done, substrate table 106 is provided with amultitude of openings on a flat uppermost surface, gas being fed throughthe openings to provide a gas cushion which is capable of supportingsubstrate 114. This is conventionally referred to as an air bearingarrangement. Substrate 114 is moved over substrate table 106 using oneor more actuators (not shown), which are capable of accuratelypositioning substrate 114 with respect to the path of beam 110.Alternatively, substrate 114 may be moved over substrate table 106 byselectively starting and stopping the passage of gas through theopenings.

Although lithography apparatus 100 according to the invention is hereindescribed as being for exposing a resist on a substrate, it will beappreciated that the invention is not limited to this use and apparatus100 may be used to project a patterned projection beam 110 for use inresistless lithography.

The depicted apparatus 100 can be used in four preferred modes:

1. Step mode: the entire pattern on the array of individuallycontrollable elements 104 is projected in one go (i.e., a single“flash”) onto a target portion 120. Substrate table 106 is then moved inthe x and/or y directions to a different position for a different targetportion 120 to be irradiated by patterned projection beam 110.

2. Scan mode: essentially the same as step mode, except that a giventarget portion 120 is not exposed in a single “flash.” Instead, thearray of individually controllable elements 104 is movable in a givendirection (the so-called “scan direction”, e.g., the y direction) with aspeed v, so that patterned projection beam 110 is caused to scan overthe array of individually controllable elements 104. Concurrently,substrate table 106 is simultaneously moved in the same or oppositedirection at a speed V=Mv, in which M is the magnification of projectionsystem 108. In this manner, a relatively large target portion 120 can beexposed, without having to compromise on resolution.

3. Pulse mode: the array of individually controllable elements 104 iskept essentially stationary and the entire pattern is projected onto atarget portion 120 of substrate 114 using pulsed radiation system 102.Substrate table 106 is moved with an essentially constant speed suchthat patterned projection beam 110 is caused to scan a line acrosssubstrate 106. The pattern on the array of individually controllableelements 104 is updated as required between pulses of radiation system102 and the pulses are timed such that successive target portions 120are exposed at the required locations on substrate 114. Consequently,patterned projection beam 110 can scan across substrate 114 to exposethe complete pattern for a strip of substrate 114. The process isrepeated until complete substrate 114 has been exposed line by line.

4. Continuous scan mode: essentially the same as pulse mode except thata substantially constant radiation system 102 is used and the pattern onthe array of individually controllable elements 104 is updated aspatterned projection beam 110 scans across substrate 114 and exposes it.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

In the embodiment shown in FIG. 1, array of individually controllableelements 104 is a programmable mirror array. Programmable mirror array104, comprises a matrix arrangement of tiny mirrors, each of which canbe individually tilted about an axis. The degree of tilt defines thestate of each mirror. The mirrors are controllable, when the element isnot defective, by appropriate control signals from the controller. Eachnon-defective element is controllable to adopt any one of a series ofstates, so as to adjust the intensity of its corresponding pixel in theprojected radiation pattern.

In one example, the series of states includes: (a) a black state inwhich radiation reflected by the mirror makes a minimum, or even a zerocontribution to the intensity distribution of its corresponding pixel;(b) a whitest state in which the reflected radiation makes a maximumcontribution; and (c) a plurality of states in between in which thereflected radiation makes intermediate contributions. The states aredivided into a normal set, used for normal beam patterning/printing, anda compensation set, used for compensating for the effects of defectiveelements. The normal set comprises the black state and a first group ofthe intermediate states. This first group will be described as greystates, and they are selectable to provide progressively increasingcontributions to corresponding pixel intensity from the minimum blackvalue up to a certain normal maximum. The compensation set comprises theremaining, second group of intermediate states together with the whiteststate. This second group of intermediate states will be described aswhite states, and they are selectable to provide contributions greaterthan the normal maximum, progressively increasing up to the true maximumcorresponding to the whitest state. Although the second group ofintermediate states are being described as white states, it will beappreciated that this is simply to facilitate the distinction betweenthe normal and compensatory exposure steps. The entire plurality ofstates could alternatively be described as a sequence of grey states,between black and white, selectable to enable grey-scale printing.

Exemplary Operation

FIG. 6 shows an arrangement of a conventional lithographic system andmethod, in which a row of controllable elements 2 is used to project apixel pattern onto a substrate. The elements are small mirrors whosetilt angles may be adjusted by appropriate control signals. The upperpart of FIG. 6 shows four of mirrors 2 w in their whitest states, andtwo of mirrors 2 b tilted into their black states. The lower part ofFIG. 6 shows, in highly simplified form, the intensity distributionalong a line on the target substrate through the centers of the pixelscorresponding to the row of elements 2. As can be seen, the intensitiesof the pixels corresponding to white elements 2 w (w=white) are at amaximum value I₀, and the intensities of the pixels corresponding toblack elements 2 b (b=black) are substantially zero. The pixels havingintensity I₀ are projected onto a white region of the target, with thetransition to black pixels being arranged at the white region edge.Thus, in this conventional method, the whitest possible (e.g., mostintense) element states are used for normal exposure of white regions.

FIG. 7 shows a first state of operation of lithographic apparatus 100according to one embodiment of the present invention, while FIG. 8 showsa second state of operation of lithographic apparatus 100 according tothe embodiment of the present invention. For example, FIG. 7 shows partof a normal exposure step (i.e., normal printing), while FIG. 8illustrates a combination of normal and exceptional control of theelements to expose a substrate and provide compensation.

In contrast to FIG. 6, FIG. 7 shows a row of elements 2 of theprogrammable mirror array of the apparatus of FIG. 1, used to performnormal substrate exposure (i.e. when no compensation for defectiveelements is required) in a method embodying the invention. Here,elements 2 g (g=gray) whose pixels fall on a white region of thesubstrate are in one of their grey states. However, a radiation sourceof sufficient power is being used such that, even with theircorresponding elements in these grey, reduced intensity states, thepixels falling on the white region still have intensity I₀. Elements 2 bwhose pixels fall on the black region of the substrate are set in theirblack states. Thus, the controller of the apparatus of FIG. 1 isarranged to control the elements such that white regions of the targetare normally exposed to pixels whose corresponding elements are set ingrey, not white, states.

By normally printing with grey pixels, an intensity headroom window of100-200% may be reserved, on top of the 0-100% used for normal printing.Thus, the apparatus of FIG. 1, with suitably arranged controller, can beused to add the intensity missed by a black dead pixel (which wasintended to print at full, normal white intensity) in a previous, orsubsequent exposure step. This compensation may conveniently be achievedwhen subsequent pixel patterns overlap pixel-to-pixel.

FIG. 8 shows the row of elements 2 from FIG. 7 again being used to printa white feature edge. This time, however, part of the white regionreceived a zero dose in a preceding exposure dose, as a result ofexposure to a dead black pixel. Thus, while three of the elements 2 gare being normally controlled, to adopt a grey state appropriate tonormal white printing, a selected element 2 w has been set to itswhitest state, such that its corresponding pixel has intensity 2I₀.

In one example, fine positioning of the projected radiation pattern maybe achieved by grey scaling, which allows for independence from the gridof mirrors.

In one example, non-defective elements may be controllable to achieve anumber, e.g. 64, of grey levels for each pixel. In this example, thecontrol system allows as close an approach as possible to the intendedgrey-level of the “dead” pixel.

An example of how “off-grid” printing can be achieved is as follows. A“white” line, two pixels wide, can be printed on-grid by setting aseries of six pixels to the following states: B B W W B B (whereB=black, W=white, and the series of six pixels runs across the printedline). To print a two-pixel-wide line exactly halfway between gridpositions (i.e. to print off-grid) the six pixels can be set to: B B G WG B (where G is a grey state, halfway between black and white).

It will be appreciated that, although FIGS. 7-8 show element arrayscomprising controllable-tilt mirrors, arrays of different controllableelements may be used in alternative embodiments. For example, pistonmirrors, or mirrors having a combination of piston and tilt function(so-called piston-tilt mirrors) may be used. With piston mirrors, ablack pixel is created by a 180 degree phase difference (e.g., ¼ λheightdifference, passed two times in reflection to give 180 degrees) betweenneighbouring mirrors. This means that the pixel on the wafer is in factobtained by the cooperative effort of two sub-resolution piston mirrors.The pixels can thus be seen as lying “in between” the mirrors. Both tiltand piston mirrors provide the freedom to project any desired pattern onthe wafer and allow for grey scaling to move the pattern over the mirrorgrid. Thus, in this example it may not be necessarily a one-to-onemapping of a pixel on the target substrate to a mirror in the array ofcontrollable elements. A pixel may be created by a combination ofelements, such as piston mirrors.

FIG. 3 illustrates part of a lithographic method, according to oneembodiment of the present invention, which can be performed withapparatus such as that shown in FIG. 1. A dosage pattern of white andblack regions is to be provided to the target surface of a resist layerof a substrate W. Part of the pattern is shown with the desired boundarybetween black and white regions being indicated by lines 13. A pattern 1of pixels is being projected onto the target surface, each pixelcorresponding to a respective controllable element in an array used topattern a radiation beam prior to its projection. For simplicity, thepattern is shown to consist of just 16 pixels. In practice, the numberof pixels may be in excess of a million. The pixels falling on the whiteregion would normally be set to one of their grey intensity values, andthose falling on the black regions would normally be set to black.However, pixel 11, which falls on the white region, corresponds to adefective element, and without compensation cannot deliver the desireddose. To compensate for this, the intensities of one or more of theneighboring pixels 10, falling on the same white region, are increasedabove the normal printing maximum. This is done by setting thecorresponding non-defective elements to one of their white compensationstates. Thus, compensation may be achieved within the white region, andwithout using pixels 12, which fall on the black regions.

It will be appreciated that the number and location of the selectedpixels, and indeed the magnitude of the increased intensity of eachselected pixel, may be calculated to give a desired compensation, anddesired edge definition. Also, while a problem with conventionalcompensation techniques was that a compensation dose to an intendedblack region could not be undone, in the above embodiment of the presentinvention, the selective overdosing at a white location in one exposurestep, to compensate for an adjacent defective pixel, can itself becompensated by reducing the dose given to the “overdosed” location in asubsequent step.

FIG. 4 illustrates pre- and post-compensation in a two-pass method,according to an embodiment of the present invention. This is in contrastto FIG. 3, which illustrates an example of substantially simultaneouscompensation using adjacent pixels in the same exposure step. In theembodiment of FIG. 4, during a first exposure step a pixel pattern 1 ais projected onto a substrate, using a corresponding array of elements.In a second step, a second pixel pattern 1 b is projected onto thesubstrate by the same array. The projected patterns overlap, but havebeen shifted so that no defective element can fall on the same part ofthe substrate twice. Again, only a small pattern (e.g., nine pixels) isshown to simplify the description. This simplification results indefective pixels from the first and second passes being adjacent eachother (as described below). It is to be appreciated that this would nottypically occur in practice, where an array may have in excess of onemillion elements, with fewer than 5 dead elements per million. A centralelement of the array is defective, such that corresponding pixels 11 a,11 b of the first and second patterns are dead black pixels. However,they both fall on a white region of the target substrate. Thus, in thefirst step, to compensate for the subsequent underexposure of the targetby dead pixel 11 b in the second step, pixel 13 a is arranged to haveincreased intensity (e.g., higher than the normal white-region printingintensity). Other pixels 10 a of the first pattern falling on the whiteregion are arranged to have the normal, lower white-printing intensity,and pixels 12 a falling on black regions are arranged to have minimumintensity. Similarly, in the second step, to compensate for thepreceding underexposure of the target by dead pixel 11 a in the firststep, pixel 13 b is arranged to have increased intensity. The remainingnon-defective pixels all fall on the white region and are arranged tohave the normal, lower white-printing intensity.

FIG. 5 illustrates a two-pass lithographic method employingpost-compensation, according to one embodiment of the present invention.Here, a first pixel pattern 1 c is projected in a first exposure step,using a first array of elements which includes a defective element,giving rise to defective pixel 11 c. A second, overlapping pixel pattern1 d is projected in a second exposure step, using a second array havingno defective elements. In the first step, pixels 12 c are black, pixels10 c have normal white intensity, and pixel 11 c is dead black, but wasintended to be normal white. In the second step, compensation forprevious dead black pixel 11 c is achieved by making pixel 13 d moreintense, as compared with normal white pixels 10 d. Pixels 12 d in thisexample are black, as they fall outside the white region.

It will be appreciated that the simultaneous, pre- and post-compensationtechniques illustrated by FIGS. 3, 4, and 5 may be used separately, ormay be combined in a single method embodying the invention.

It will also be appreciated that in embodiments of the invention,although radiation doses greater than the normal printing doses are usedfor compensation, this does not preclude the additional use of normalprinting doses (e.g., delivered by black or grey pixels) to contributeto the overall compensation. In general, the states of all of thefunctioning elements of an array are set, taking into account the knowndefective elements, to give a dosage pattern (e.g., intensitydistribution). The given dosage pattern is set so that is optimallycompensates for the effects of defective elements, while giving uniformfeature sharpness (i.e., uniform dosage slope at feature edges, givinguniform edge definition). This uniformity is important so that nominallyidentical devices, formed at different positions on the substrate, willhave the same characteristics (e.g., clocks running at the same speed).Thus, the element states are controlled to achieve compensation which,at a particular point, may not give the sharpest possible feature edge,but which results in uniform feature definition over the substrate.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A device manufacturing method, comprising: (a)patterning a beam of radiation with a patterning device to form aplurality of pixels; (b) projecting the patterned beam onto a targetportion of a substrate; (c) controlling the patterning device, such thateach pixel in the plurality of pixels delivers a first radiation dose tothe target portion that is below or equal to a first value; and (d)controlling the patterning device, such that at least one selected pixelin the plurality of pixels delivers a second radiation dose to thetarget that is greater than the first value to compensate at leastpartially for: (1) an effect of a defective area in the patterningdevice on a pixel in the plurality of pixels adjacent the selectedpixel, or (2) an underexposure of the target portion at a locationcorresponding to the selected pixel resulting from exposure of thelocation to a pixel in the plurality of pixels affected by a knowndefective area of the patterning device.
 2. The method of claim 1,wherein step (c) comprises using a value for the second radiation dosethat is a factor of about 1.1 to 2.0 of the first value.
 3. The methodof claim 1, further comprising: delivering a dosage pattern onto thesubstrate, the dosage pattern comprising, white regions onto which aradiation dosage at least equal to a threshold value is delivered, andblack regions onto which a radiation dosage less than the thresholdvalue is delivered, wherein step (c) comprises controlling thepatterning device such that each pixel in the plurality of pixelsprojected onto one of the white regions delivers a radiation dose nogreater than the threshold value; and wherein step (d) comprisescontrolling the patterning device such that the selected pixel isprojected onto a white region and delivers an increased radiation doseto the white region to compensate at least partially for at least oneof: an effect of a defective area of the patterning device on a pixeladjacent the selected pixel on the same white region, and anunderexposure of the white region at the location of the selected pixelresulting from exposure of the location to a pixel affected by adefective area in the patterning device in another exposure step.
 4. Themethod of claim 3, wherein: the substrate comprises a layer of radiationsensitive material having an activation threshold; the threshold valueis substantially equal to the activation threshold; and the targetsurface is a surface of the layer.
 5. The method of claim 1, furthercomprising: performing steps (c) and (d) first and second times todeliver a radiation dosage pattern to a target area of the substrate,wherein, during performing of steps (c) and (d) the first time, thetarget area is exposed to a first plurality of the pixels correspondingto a first portion of the patterning device, and during performing ofsteps (c) and (d) the second time, the target area is exposed to asecond plurality of the pixels corresponding to a second, differentportion of the patterning device.
 6. The method of claim 5, furthercomprising: controlling the first portion of the patterning device suchthat the first plurality of pixels includes at least one selected pixelarranged to deliver an increased radiation dose to compensate at leastpartially for an underexposure effect of a defective element of thefirst or second portions of the patterning device.
 7. The method ofclaim 5, further comprising: controlling the second portion of thepatterning device such that the second plurality of pixels includes atleast one selected pixel arranged to deliver an increased radiation doseto compensate at least partially for an underexposure effect of adefective element of the first or second portions of the patterningdevice.
 8. The method of claim 1, wherein each pixel corresponds to anarea of the patterning device.
 9. The method of claim 1, wherein step(d) comprises controlling at least one area immediately adjacent thedefective area, such that at least one corresponding pixel, immediatelyadjacent the pixel corresponding to the defective area, delivers anincreased radiation dose to compensate at least partially for the effectof the defective area.
 10. The method of claim 1, further comprising:controlling each non-defective area to selectively adopt one of at leastthree states, the states comprising, a nominal black state, in which thearea interacts with the beam so as to make a minimum contribution to theradiation dose delivered by a corresponding pixel; at least one nominalgrey state, in which the area interacts with the beam so as to make anincreased contribution to the radiation dose delivered by thecorresponding pixel; and at least one nominal white state, in which thearea interacts with the beam so as to make a contribution to theradiation dose delivered to the corresponding pixel greater than that inany grey state, wherein step (c) comprises controlling each area toadopt one of the black or grey states, wherein step (d) comprisescontrolling at least one selected area to adopt a white state.
 11. Themethod of claim 1, further comprising performing an exposure step usingsteps (c) and (d).